1. Technical Field
This disclosure relates to redox flow battery systems for energy storage employing multi-cell electrochemical reactors composed of sequences of “filter press” assembled planar elements to form a battery stack and a method of monitoring operating conditions and controlling the functioning of every electrode of the multi-cell stack.
2. Description of Related Art
Redox flow battery systems are increasingly attracting interest as efficient energy conversion systems. Among the numerous redox couple candidates, the all vanadium redox system being one of the most preferred.
Structurally, the electrochemical reactors that have been proposed for redox flow battery systems, have been derived from the electrochemical reactor structures developed for general electrolysis processes, the only adaptation having concerned the materials employed as electrodes.
The documents GB-A-2,030,349, U.S. Pat. No. 4,786,567, WO99/39397, WO01/03213, WO01/03224, WO01/76000, WO02/15317, WO03/003483, WO03/007464, WO03/043170, WO2004/079849, EPRI, Technical Update Report, “Vanadium Redox Flow Batteries” (An In-depth analysis), ©-2007—Electric Power Research Institute, Inc., US-2012/0156535-A1; WO2012/001446, WO2012/020277, WO2012/032368, WO2012/042288, and in particular the article: “State of charge monitoring methods for vanadium redox flow battery control”, Maria Skyllas-Kazacos, Michael Kazacos, School of Chemical Engineering, University of New South Wales, Sydney, NSW 2052, Australia, Journal of Power Sources: 196 (2011) 8822-8827, offer an ample review of the state of the art and a discussion of specific instrumental means of control of all-vanadium redox flow systems.
Typically, in redox flow storage systems besides absence of gas evolution (H2, O2 or other elements) at the electrodes under correct operating conditions, apart from minor accidental parasitic occurrences that must be absolutely be prevented or held at negligible levels, the electrodes themselves are chemically inert (i.e. do not undergo any modification at their surface wetted by the electrolyte solution). These conditions make mass transport of the redox process supporting ions to active electrode sites a most critical parameter, together with electrochemical and physical characteristics of the wetted surface of the electrode material, that affects the dynamics of ion oxidation and reduction at the respective electrodes of the electrochemical cell. For many of these systems, for example for the all vanadium (V/V redox flow cell systems and similar systems (Fe/V, V/Br, Cr/Fe, Zn/Ce, Polysulfide/Br), for economically acceptable current densities to be supported, porous and fluid permeable electrodes are necessary.
Moreover, chemical inertness of the electrode materials that need to be retained when switching from cathodic polarization to anodic polarization during a cycle of charging and discharging of the redox storage system, and the requisite of having a relatively high H+ discharge overvoltage when positively polarized in respect to the electrolyte solution and a high OH− discharge overvoltage when negatively polarized in respect to the electrolyte solution obliges to use carbon base electrodes.
A typical stack cell assembly contemplates a fluid impervious perm-ionic membrane cell separator, identical porous and fluid permeable carbon felt electrodes on both sides of the membrane separator in electrical contact with respective carbon base electrically conductive back plates, defining, together with nonconductive frames (commonly made of plastic), respective flow compartments of the positive electrolyte solution and of the negative electrolyte solution, respectively.
The conductive back plate most often is an inter-cell separator element according to the common architectural approach of a bipolar stack of a plurality of cell in electrical series between the two end elements. According to an alternative architectural approach (WO2004/079849), the conductive back plates of a multi-cell stack assembly separate the flow compartments of same sign of a plurality of interleaved, two face electrodes, electrically connected in parallel.
Mass transport to the electrodes must be assisted by a forced flow of the two electrolyte solutions through the respective porous electrode compartments. Of course, the pumping of electrolyte solutions represents “passive power” that significantly detracts from the overall power yield of every complete cycle of energy storage.
Generally, redox flow storage systems because of their peculiarity of not directly tying storage capacity to the size of the electrochemical reactors, are ideally suited for large storage facilities of electrical grid operators that must manage an increasingly great quota of discontinuous renewable energy sources connected to the grid like photovoltaic and wind power generators.
Nevertheless, large power ratings inevitably call for large cell (i.e. projected electrode area) being the maximum current density of operation of the cells limited by factors affecting the dynamics of ion charge and discharge reactions at the electrodes and the internal voltage drop due to the electrical cell resistance that tends to steeply increase at excessively high current densities.
The forced flow rate of the electrolyte solutions is increased when the current density increases or when the cell voltage drops in order to enhance the reaction dynamics at the electrodes by increasing irrigation of the porous mass of the partly compressed carbon felts though spending more power (passive power).
Normally the electrolyte solution enters the cell compartment through one or more inlet ports distributed along one side of a generally rectangular nonconductive frame or a nonconductive (unloaded plastic) frame portion of a molded back plate having a central portion (cell area) made of a moldable carbon loaded conductive aggregate, and exits the flow compartment through one or more outlet ports distributed along the opposite side of the nonconductive frame. The pump assisted circulation forces the solution through the pervious carbon felt electrode that substantially fills the whole cell area in order not to leave any by-pass flow paths unobstructed by the partly compressed felt.
A partial compression of the felt electrode between the perm-ionic membrane cell separator and the electrically conductive back plate, though increasing pressure drop, remains necessary for maintaining ad adequate electrical contact over the whole cell area that should promote a substantial equipotentiality of the working electrode, notwithstanding attempts to provide for a good contact in other ways.
Incidence of so many contrasting requisites and severe constraints on the choice of usable conductors because of electrochemical and chemical resistance considerations, has left the practitioner battling with the intrinsic non homogeneity of the compressed felt in terms of permeability (resistance to the liquid stream) that inevitably creates preferential flow paths through the porous electrode mass leaving portions of the electrode become “starved” of reducible (or oxidable) ions causing other portions to work at proportionately incremented current density and thence begin themselves to starve, in a unpredictably varying fashion. Over pumping the electrolyte solutions, besides dramatically lowering energy efficiency, seldom cures the problem and under certain conditions may even become ineffective.
Dramatically lowering the maximum rated current density of the cells may significantly lessen these problems, but the increased cell area requirement that has a major impact on investment may render competitively uneconomical the choice of a redox flow system.
Overcharging of negative electrolyte solution causes evolution of hydrogen gas, overcharging of the positive electrolyte causes evolution of oxygen that is destructive for the carbon felt electrode, moreover commonly used permionic membranes stem to an inevitable volumetric and ionic unbalance of the distinctly circulated solutions requiring periodic re-balancing of the two circuits.
Many instruments have been proposed to provide information on the conditions the electrochemical processes at the carbon felt electrodes take place, among which may be mentioned:                a) operation voltage of the individual cell (providing for dedicated external electrical voltage probe wires contacting each inter-cell conductive plate, in case of bipolar cell stacks) by ordinary voltmeters or equivalent instruments. A practice viable for test, but hardly compatible in commercial plants because is of associated risk of corrosions and/or leaks with such a wiring;        b) electrical current through the cells working in series by ordinary current meters or equivalent instruments;        c) open circuit voltage of cells, typically by circulating the two electrolyte solutions in a measuring cell;        d) state of charge of the electrolyte solutions by measuring the voltage difference between a probe electrode immersed in the electrolyte solution and a reference electrode immersed in a reference solution at substantially null state of charge, or by colorimetric analysis of the electrolyte solution, or by electrical conductivity measurements;        e) modifications of mass balance of solvent and ionic species in the two distinct electrolyte solutions, by chemical analysis.        
So far even the most generous deployment of sophisticated instruments and techniques of assessment of electrical, chemical and electrochemical parameters that affects the charging and discharging processes taking place in the cells has failed to provide reliable information to eliminate the attendant risks of erratic failures of single cell of a multi cell stack, nor of observing erratic drops of overall efficiency, notwithstanding deliberate increases of the pumping rate, beyond any design limit, through the respective flow compartments of the cells. Phenomena these that tends to increasingly occur and to worsen from the moment of putting in operation the multi cell stack.
Commercially operated systems traditionally use bipolar cell stack assemblies. This normally impedes to monitor disuniformities of cell voltages or flow rates through the flow compartments of the single cell that may occur in any of the numerous cells in series of a bipolar cell stack. Any malfunctioning at felt electrode or membrane level is hardly detectable and too often stems to a failure of the whole stack. State of charge of the electrolyte solutions is normally monitored for the whole bipolar stack and with relatively complex implements that are hardly deployable at single cell level.